Technical Field
The present invention relates to a telecentric optical apparatus having a telecentric lens provided on both the object side and the image side.
Background Art
Telecentric optical apparatuses having a telecentric lens provided on both the object side and the image side have a feature to the effect that, even when an object moves in the optical axis direction (the Z-direction), the size of the image remains unchanged in the X- and Y-directions, which are orthogonal to the Z-direction. For this reason, telecentric optical apparatuses are employed as optical systems for linear scales and image measuring apparatuses, and are widely used for, for example, wide-field one-time measurements, or the like, of objects with steps.
JP3708845B discloses a bi-telecentric optical system provided with a front group, which has positive refractive power as a whole, and a rear group, which has positive refractive power as a whole, wherein the back focus of the front group and the front focus of the rear group are arranged such that they coincide with each other, and an aperture stop is arranged at the coinciding focus position. Since various aberrations and the telecentricity (the angle of principle ray) of the main light beam of an off-axis luminous flux are well corrected, such bi-telecentric optical system is suitably used as an objective lens of an image process measuring device.